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Triggering Modes of TRIAC
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Mode 1 : When the terminal $\mathrm{M}_{1}$ is positive with respect to $\mathrm{M}_{2}$ and a positive gate voltage is applied; the TRIAC behaves like a conventional thyristor. In this mode, the cathode is connected to the $n_{2}-$ layer, the anode is connected to the $p_{1}$ -layer, and the $p_{2}$ -layer acts as a gate as shown in Fig.1(a). Since junctions $J_{4}$ and $J_{5}$ are reverse biased, and therefore inactive, the main current flows through the right side of the $p_{1}-n_{1}-p_{2}-n_{2}$ section.

Mode 2 : In the second mode of operation (Fig.1(b)), the device is switched on by applying a negative gate voltage with respect to $\mathrm{M}_{2}$ . In this mode, the device goes into the conducting state by an operation called junction gate operation. The junction $J_{4},$ between the $n_{3}$ region and the $p_{2}$ region is now forward biased, and free charge carriers (electrons) are injected from the $n_{3}$ region to the $p_{2}$ region. Initial conduction takes place between $p_{1}-n_{1}-p_{2}-n_{3}$ , due to an increase in the gain of transistor $\mathrm{n}_{3}-\mathrm{p}_{2}-\mathrm{n}_{1}$ . Full conduction of this transistor, results in the current flowing out of the device and towards the $n_{2}$ region. This lateral current then provides the required gate current and triggers the $\mathrm{p}_{1}-\mathrm{n}_{1}-\mathrm{p}_{2}-\mathrm{n}_{2}$ thyristor.

Mode 3 : In this mode $\mathrm{M}_{1}$ is negatively biased and a positive gate voltage is applied with respect to $\mathrm{M}_{2}$ . The junction $J_{3}$ becomes forward biased between $\mathrm{M}_{2}$ and the shorted gate terminal. Free electrons are then injected from the $n_{2}-$ layer to the $p_{2}$ -layer and diffused to the $n_{1}-$ layer, resulting in an increase of the forward bias of junction $J_{2} .$ In this mode the junction $J_{1}$ is reverse biased which can be broken-down by increasing the carrier concentration in layer $n_{1}$ . Consider the transistor formed by the $\mathrm{n}_{2}-\mathrm{p}_{2}-\mathrm{n}_{1}$ layers. Since the gate is made positive with respect to $\mathrm{M}_{2}$ terminal, the $\mathrm{n}_{2}-\mathrm{p}_{2}-\mathrm{n}_{1}$ transistor will be properly biased and a positive base current will flow into the layer $p_{2} .$ This will increase the carrier concentration in layer $n_{1},$ which leads to the breakdown of the reverse biased junction $J_{1}$ . The junction $J_{4}$ ( Fig.1(c)) is reverse biased and is inactive. Thus, full conduction takes place through the left-side $\mathrm{p}_{2}-\mathrm{n}_{1}-\mathrm{p}_{1}-\mathrm{n}_{4}$ thyristor.

Mode 4: Fig.1(d) shows the fourth mode of operation. In this mode, the gate is negative with respect to the $\mathrm{M}_{2}$ terminal. Due to forward biasing of junction $J_{4},$ triggering is initiated by injection of electrons from the $n_{3}$ to the $n_{1}$ region. This lowers the potential at $n_{1},$ which causes holes to be injected from the $p_{2}$ region to the $n_{1}$ region and provides the base current for the $\mathrm{p}_{2}-\mathrm{n}_{1}-\mathrm{p}_{1}$ transistor. Eventually, the left-side $\mathrm{p}_{2}-\mathrm{n}_{1}-\mathrm{p}_{1}-\mathrm{n}_{4}$ transistor turns on. Since the junction $J_{3}$ is reverse biased, the device current flows from the $n_{4}$ region.

Although the TRIAC may be turned on by any of the four methods stated above, it is more sensitive in the case of a positive gate current.

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